The aerial fluid, discovered by Dr. Black, was one of those most commonly met with. He called it _fixed air_, from its property of adhering or fixing itself to different bodies. It was found to be the same with that which had been discovered by Van Helmont, and by him named _gas sylvestre_ (spirit of wood)[64] or the fume of charcoal; it was found to be the same with the steam of fermenting liquor, and with that very frequent and dangerous vapour, met with in coal mines, called in Scotland the _choke-damp_. Like other discoveries, this was quickly pushed beyond its proper bounds, and applied to the solution of phenomena which it could not solve. Dr. MBride, particularly, supposed it to be the bond of union between the particles of matter, or in other words the principle of cohesion itself. It was also supposed to be the substance of those scorching winds, called _samiel_, met with in Asia and Africa, and which sometimes prove fatal to travellers. The pernicious vapours called _mofetes_, which sometimes issue from the old lavas of Vesuvius in Italy, were likewise supposed to be the same;[65] but of this, particularly with regard to the samiel, there seems to be no sufficient evidence. [64] This must be understood only of its general properties and effects; for, though the fume of charcoal possesses many of the apparent properties of pure fixed air, it contains also a very considerable quantity of another kind of gas. [65] Many fabulous stories have been related concerning the samiel. Even so late a traveller as Mr. Ives has adopted some of those exaggerated accounts which have been discredited by those who have long resided in the countries where this wind is commonly met with. It is not peculiar to the deserts of Arabia, but is met with in all hot countries which are destitute of water. In the African deserts therefore it is common; and Mr. Bruce describes it by the name of _simoom_. It was preceded by whirlwinds of a very extraordinary kind. “In that vast expanse of desert (says he) from W. and to N. W. of us, we saw a number of prodigious pillars of sand at different distances, at times moving with great celerity, at others walking on with a majestic slowness. At intervals we thought they were coming in a very few minutes to overwhelm us; and small quantities of sand did actually more than once reach us. Again they would retreat so as to be almost out of sight; their tops reaching to the very clouds.* There the tops often separated from the bodies; and these, once disjoined, dispersed in the air, and did not appear more. Sometimes they were broken near the middle, as if struck with a large cannon shot. About noon they began to advance with considerable swiftness upon us, the wind being very strong at north. Eleven of them ranged along side of us at about the distance of three miles. The largest of them appeared to me at that distance to be about ten feet diameter....